1. Field of the Invention
The invention relates to an electronic ballast for supplying a load, for example a lamp, in particular a gas discharge lamp, with an a.c. voltage the frequency of which is higher than the mains frequency, having a rectifier to be connected to the a.c. mains, an inverter comprising two switches connected downstream of the rectifier, which inverter generates the higher frequency a.c. voltage for the load, and a harmonics filter arranged between the rectifier and the inverter, in the form of a clocked switching controller.
In particular the invention relates to an electronic ballast, the electronic harmonics filter of which consists of a clocked switching controller in the form of an inverting transformer (inverse transformer).
2. Description of the Related Art
Gas discharge lamps have an unsatisfactory light efficiency and tend to flicker when operated with mains frequency. For this reason they are usually operated with electronic ballasts which work at mains voltage and generate a higher frequency operating voltage for the gas discharge lamps therefrom, the frequency of which lies in the region from 20 to 50 kHz. The electronic ballasts have a rectifier, which rectifies the mains voltage and an inverter, which generates the higher frequency operating voltage from the rectified mains voltage. Through the utilization of electronic ballasts the light efficiency of the gas discharge lamps may be considerably increased and flickering may be avoided. At the same time, however, the problem occurs that as a result of the rectification harmonic waves result which cause unacceptable disturbances when fed back into the mains. To avoid this, a passive filter can be connected to the rectifier and the mains which filter must however comprise a relatively large and expensive choke. This choke can be avoided if an electronic harmonics filter is connected between rectifier and inverter instead of the passive filter. The electronic harmonics filter, which is usually constructed in the form of a clocked switching controller, manifests itself in that it removes pulse-like current from the rectifier, whereby the envelope curve of the current pulses is approximated to a sinusoidal half-wave form with mains frequency.
Such a harmonics filter is described for example in EP 268 043 A1 or in the book by C. H. STURM/E. KLEIN, "Betriebsgerate und Schaltungen fur Lampen" ("Operating devices and circuits for lamps"), Publisher Siemens AG, 6th Edition, 1992, pages 127/128. The electronic harmonics filter consists of a clocked switching controller; which can for example be a step-up transformer (booster) or an inverting transformer. The principle of such a clocked switching controller is also described in the book by U. STIETZE, CH. SCHENK, "Halbleiterschaltungstechnik" ("Semiconductor circuitry techniques"), Publisher Springer, 9th Edition, pp. 563 to 570. With the known harmonics filter the output voltage of the switching controller is constantly monitored and compared with a fixed desired-value voltage. From the control difference, the controller generates a control signal of variable switching frequency for the switch of the switching controller.
The combination of the known component circuits provides for the electronic ballast a circuitry configuration consisting, in terms of function, of a rectifier, an electronic harmonics filter and an inverter. Hereby an electronic harmonics filter structured in such a way proves to be, in terms of circuitry configuration, too complicated and to result in unacceptable manufacturing costs, in particular when used for individual lamps.
There were therefore suggested electronic ballasts with harmonics filters in the form of step-up transformers (boosters) in which the control switch of the clocked switching controller forms at the same time one of the two inverter switches. Such circuits are known for example from U.S. Pat. No. 5,224,025 or EP-A-O 435 628.